Many devices utilize current and/or charge measurements for operations. One such set of devices are digital cameras that utilize detector arrays to acquire scene energy. These detector arrays typically include detector cells that absorb scene energy. The voltages on these cells can then be measured, quantified and processed in order to produce a digital image of the scene being viewed. As part of this process, a detection current is often measured with respect to each pixel cell within an image sensing system.
One method of measuring a current is to integrate that current on a capacitor for a fixed amount of time and then measure the voltage on the capacitor. Because the voltage is inversely proportional to the capacitance, a small capacitance will give the circuit higher sensitivity (i.e., a large change in voltage for a small change in current). Unfortunately, small capacitors have more limited integration times and, therefore, cause reduced signal to noise ratios. In other words, the longer the integration time is for the integration node, the better the signal to noise ratio tends to be. At long integration times, however, the voltage on the capacitor also gets large, and larger capacitors tend to degrade sensitivity. Thus, although larger integration capacitors can increase the integration time, they also have the downside of lowering sensitivity. And although higher integration voltages increase integration times, these higher voltages also cause problems for current integrated circuits where the maximum voltages are limited to avoid damage to the integrated devices. Other attempts to improve integration have included subtracting a fixed current from the integration node. This technique, however, can increase noise and/or become very complicated to achieve.